The industrial scale preparation of polyisocyanates by reacting the corresponding amines with phosgene has long been known from the prior art, the reaction being conducted in the gas or liquid phase and batchwise or continuously (W. Siefken, Liebigs Ann. 562, 75-106 (1949)). There have already been multiple descriptions of processes for preparing organic isocyanates from primary amines and phosgene; see, for example, Ullmanns Encyklopadie der technischen Chemie [Ullmann's Encyclopedia of Industrial Chemistry], 4th ed. (1977), volume 13, p. 351 to 353, and G. Wegener et al. Applied Catalysis A: General 221 (2001), p. 303-335, Elsevier Science B. V. There is global use both of aromatic isocyanates, for example methylene diphenyl diisocyanate (MMDI—“monomeric MDI”), polymethylene polyphenylene polyisocyanate (a mixture of MMDI and higher homologs, PMDI, “polymeric MDI”) or tolylene diisocyanate (TDI), and of aliphatic isocyanates, for example hexamethylene diisocyanate (HDI) or isophorone diisocyanate (IPDI).
Modern industrial scale preparation of polyisocyanates is continuous, and the reaction is conducted as an adiabatic phosgenation as described in EP 1 616 857 B2. Unwanted deposits and by-products in the reactor are avoided through correct choice of reaction temperature and pressure. In the mixing space, a molar excess of phosgene relative to the primary amino groups should be ensured. A three-stage phosgenation line is described in EP 1 873 142 B1, in which the pressure between the first stage of a dynamic mixer and the second stage of a first phosgenation reactor remains the same or rises and, in the third stage, in an apparatus for phosgene removal, the pressure is lower than in the second stage.
WO 2013/029918 describes a process for preparing isocyanates by reacting the corresponding amines with phosgene, which can also be conducted at different loads on the plant without any problems, and more particularly, even when running the plant in the partial load range, the mixing and/or the reaction is said to proceed within the optimized residence time window in each case, by increasing the ratio of phosgene to amine or adding one or more inert substances to the phosgene and/or amine stream. The process of the invention is to enable operation of an existing plant at different loads with constant product and process quality. This is to dispense with the provision of several plants with different nameplate capacities.
The application teaches that essential parameters of a phosgenation, such as the residence times of the co-reactants in the individual apparatuses in particular, are optimized for the operation of the production plant at nameplate capacity, which can lead to problems in terms of yield and product purity when the plant is operated at lower than nameplate capacity (cf. page 2 lines 20 to 36). In order to be able to attain the optimized narrow residence time windows even at partial load (i.e. reduced amine flow rate compared to operation at nameplate capacity), it is suggested that either the phosgene stream and/or the inert fraction be increased (cf. page 3 lines 5 to 19), preferably in such a way that the total flow rate of all components corresponds essentially to that at nameplate capacity (cf. page 6 lines 4 to 8). The application does mention startup and shutdown operations in the description of the background of the invention claimed on page 2, but does not disclose either any technical teaching as to the specific actions by which a non-operational production plant (i.e. amine flow rate and phosgene flow rate equal to zero) is most advantageously brought to the desired operating state of the nameplate capacity nor any technical teaching as to the specific actions by which an operational production plant is most advantageously shut down (i.e. amine flow rate and phosgene flow rate equal to zero). The technical measures disclosed in the application (i.e. the increase in the phosgene flow rate and/or the inert fraction) should be considered exclusively in the context of the problem of operation (i.e. the amine flow rate is significantly greater than zero) of a production plant at lower than nameplate capacity, and of the problem of how a plant operated at nameplate capacity can advantageously be switched to operation at lower than nameplate capacity (see the examples). The document does not address the sequence of startup of individual streams in the startup operation or the shutdown of individual streams in the shutdown operation.
The reaction output from the phosgenation line can be worked up as described in EP 1 546 091 B1. The workup of the reaction product is effected in a layer evaporator, preferably a falling-film evaporator, in which phosgene and HCl are evaporated gently.
U.S. Pat. No. 5,136,087 (B) likewise describes the removal of phosgene from the reaction mixture of the phosgenation by means of an inert solvent vapor which may originate from the solvent recovery in the phosgenation plant.
One possible embodiment of the solvent removal and recovery is described in EP 1 854 783 A2. Di- and polyisocyanates of the diphenylmethane series (MDI) which have been obtained by reacting corresponding amines dissolved in a solvent with phosgene are first freed of hydrogen chloride and excess phosgene, and then a distillative separation of this crude solution into isocyanates and solvent is conducted. The solvent is recycled into the process to prepare solutions of the feedstocks of the polyisocyanate preparation. In the case of preparation of MDI using monochlorobenzene as solvent, this distillative separation can advantageously be effected in such a way that the crude isocyanate solution is worked up in two steps to give a bottom product containing at least 95% by weight of isocyanate, based on the weight of the isocyanate-containing stream, and this bottom product is subsequently preferably freed of low boilers in further steps. In the first step, 60%-90% of the solvent present in the crude isocyanate solution is removed, preferably by a flash distillation at absolute pressures of 600-1200 mbar and bottom temperatures of 110° C.-170° C., the vapors being worked up in a distillation column having 5-20 plates and 10%-30% reflux, so as to achieve a solvent-containing stream having a diisocyanate content of <100 ppm, preferably <50 ppm, more preferably <20 ppm, based on the weight of the solvent-containing stream. In the second step, the remaining solvent is removed down to a residual content of 1%-3% by weight in the bottom product at pressures of 60-140 mbar absolute and bottom temperatures of 130° C.-190° C. The vapors can likewise be worked up in a distillation column having 5-20 plates and 10%-40% reflux, so as to achieve a solvent-containing stream having a diisocyanate content of <100 ppm, preferably <50 ppm, more preferably <20 ppm, based on the weight of the solvent-containing stream, or this stream, after condensation, is recycled back into the first distillation step as feed. In the same way, the distillate streams removed in the subsequent steps can be recycled back into the first distillation step as feed.
Given suitable design of the distillation, the recycled solvent has the aforementioned diisocyanate contents. In addition, through use of suitable technical measures, it is possible to further increase the solvent quality with regard to diisocyanate content by, for example, wholly or partly removing diisocyanate-containing solvent mist or droplets in the vapors of the one-stage or multistage distillative solvent removal by means of a demister, baffle plate or hydrocyclone, or by quenching (spraying) with fresh or recycled solvent. Combinations of the aforementioned measures are also possible.
EP 1 854 783 A2 describes the quality demands that exist for a solvent for a process for preparing polyisocyanates. It has been found that the purity of the circulated solvent which is used for preparation of the amine solution used in the phosgenation is of crucial significance for the by-product formation in the crude isocyanate. Even a content of only 100 ppm phosgene or 100 ppm of diisocyanate, based on the weight of the solvent, leads to detectable by-product formation in the crude isocyanate. While this leads to a reduction in yield in the case of distilled isocyanates, i.e. in the case of the isocyanates obtained as top product, this causes an unwanted effect on the quality (color) and reaction characteristics in the case of the isocyanates obtained as bottom product, for example the di- and polyisocyanates of the diphenylmethane series. This is detectable, for example, via chlorinated secondary components and an elevated iron content.
Carbon tetrachloride as solvent impurity gets into the phosgenation circuit via the phosgene and accumulates in the solvent through the solvent circuit. With time, the concentration of carbon tetrachloride settles at a uniform level shaped by the losses of carbon tetrachloride via the discharge with the offgas. According to the process conditions, a solvent used in the phosgenation which has not been supplied fresh but comes from recycling streams within the process has a content by mass of carbon tetrachloride of 0.01% to 5%, and in some circumstances even up to 20%, based on the total mass of the solvent including all impurities.
DE-A-19942299 describes a process for preparing mono- and oligoisocyanates by phosgenating the corresponding amines, wherein a catalytic amount of a monoisocyanate is initially charged in an inert solvent together with phosgene, the amine is added, normally dissolved in the solvent, and the reaction mixture obtained is reacted with phosgene. The intermediate formation of sparingly soluble suspensions is avoided. The desired isocyanate, in the case of full conversion of the amine, is formed in high yields and high selectivity within distinctly shortened reaction times, without formation of symmetrically substituted N,N′-urea from the amine as by-product. However, the process is comparatively complicated and energy-intensive, particularly through use of the additional monoisocyanate which has to be removed again at a later stage.
Apart from a few exceptions, the prior art described is concerned only with processes in normal operation. Startup operations until attainment of a steady operating state at the desired target flow rate of the amine (called the “startup time”) or shutdown operations until attainment of complete shutdown (called the “shutdown time”) are not considered in the documents relating to continuous industrial scale processes. Only in documents in which batchwise phosgenation is described are startup phases given more detailed consideration; see, for example, U.S. Pat. No. 2,908,703 and U.S. Pat. No. 2,822,373. Unexpected downtime (for example an abruptly forced shutdown of the plant) also lead at short notice to process regimes which can differ significantly from those in normal operation.